avformat/4xm: Make audio_frame_count 64bit

Fixes: signed integer overflow: 2099257366 * 2 cannot be represented in type 'int'
Fixes: 27486/clusterfuzz-testcase-minimized-ffmpeg_dem_FOURXM_fuzzer-5112179134824448

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/4xm.c

@@ -299,7 +299,7 @@ static int fourxm_read_packet(AVFormatContext *s,
     unsigned int track_number;
     int packet_read = 0;
     unsigned char header[8];
-    int audio_frame_count;
+    int64_t audio_frame_count;
 
     while (!packet_read) {
         if ((ret = avio_read(s->pb, header, 8)) < 0)